I think you and those males in question may be outwitting each other to the point that it ends with this disappointment.
Take a look at it from the single male's perspective:
He's straight, and he's been on the ad site for a while with very little if any success so far.
Suddenly, a couple shows interest. They're listed as straight, so that's cool. He's going to help a husband please his wife for a night. Excellent!
After a little internet conversation, it's looking even more promising. She is interested! Now he's getting excited about this possibility. I'm going to have fun with a couple. Hot damn!
Then, she asks the question: "How do you feel about MM play?"
Hmmm. He thinks they are undercover, and want to incorporate some bi play. He's hit the "Whatever it takes" stage, and, whether he intends to play with the male on some level, or try to get away without doing that, he's going to say "I'll do it! I'll do anything dammit!".
So, he falls into the trap.
Now you're pissed that he said he'll do MM play.
He's wondering what in the hell he did wrong.
If someone is listed as straight, we assume they are straight, and if they aren't straight, they can play that way.
Good luck with your future attempts at a hookup!